No abstract

These findings indicate that signalling of cGKIbeta via IRAG is an essential functional part for regulation of smooth muscle tone and of intracellular calcium by NO (exogenously applicated or endogenously synthesized) and by ANP. IRAG signalling does not modulate basal tone but might be important for blood pressure regulation under pathophysiological conditions.

The metabolic sensor adenosine-monophosphate-activated kinase (AMPK) detects the cellular energy status and adjusts metabolic activity according to the cytosolic AMP to ATP ratio. Na(+) absorption by epithelial Na(+) channels (ENaC) is a highly energy-consuming process that is inhibited by AMPK. We show that the catalytic subunit alpha1 of AMPK inhibits ENaC in epithelial tissues from airways, kidney, and colon and that AMPK regulation of ENaC is absent in AMPKalpha1-/- mice. These mice demonstrate enhanced electrogenic Na(+) absorption that leads to subtle changes in intestinal and renal function and may also affect Na(+) absorption and mucociliary clearance in the airways. We demonstrate that AMPK uses the ubiquitin ligase Nedd4-2 to inhibit ENaC by increasing ubiquitination and endocytosis of ENaC. Thus, enhanced expression of epithelial Na(+) channels was detected in colon, airways, and kidney of AMPKalpha1-/- mice. Therefore, AMPKalpha1 is a physiologically important regulator of electrogenic Na(+) absorption and may provide a novel pharmacological target for controlling epithelial Na(+) transport.

Previous in vitro studies suggested that Cl(-) currents produced by the cystic fibrosis transmembrane conductance regulator (CFTR; ABCC7) are inhibited by the alpha1 isoform of the adenosine monophosphate (AMP)-stimulated kinase (AMPK). AMPK is a serine/threonine kinase that is activated during metabolic stress. It has been proposed as a potential mediator for transport-metabolism coupling in epithelial tissues. All previous studies have been performed in vitro and thus little is known about the regulation of Cl(-) secretion by AMPK in vivo. Using AMPKalpha1(-/-) mice and wild-type littermates, we demonstrate that phenformin, an activator of AMPK, strongly inhibits cAMP-activated Cl(-) secretion in mouse airways and colon, when examined in ex vivo in Ussing chamber recordings. However, phenformin was equally effective in AMPKalpha1(-/-) and wild-type animals, suggesting additional AMPK-independent action of phenformin. Phenformin inhibited CFTR Cl(-) conductance in basolaterally permeabilized colonic epithelium from AMPKalpha1(+/+) but not AMPKalpha1(-/-) mice. The inhibitor of AMPK compound C enhanced CFTR-mediated Cl(-) secretion in epithelial tissues of AMPKalpha1(-/-) mice, but not in wild-type littermates. There was no effect on Ca(2+)-mediated Cl(-) secretion, activated by adenosine triphosphate or carbachol. Moreover CFTR-dependent Cl(-) secretion was enhanced in the colon of AMPKalpha1(-/-) mice, as indicated in Ussing chamber ex vivo and rectal PD measurements in vivo. Taken together, these data suggest that epithelial Cl(-) secretion mediated by CFTR is controlled by AMPK in vivo.

The author wishes to apologize for inadvertently providing the wrong image for Fig. 2a. Here is the correct image:The online version of the original article can be found at http://dx.doi.